modern internal combustion engines increasingly have adjustable-phase camshafts. This makes possible variable control of intake and exhaust valves, and allows the combustion chamber charge to be maximized over a wide rotation speed range of the internal combustion engine, more power being attained as compared with ordinary systems, with favorable emissions values. At the same time, residual gas can be controlled over a wide range by way of a valve control system, so that in some cases an external EGR valve can be omitted.
For reliable operation of the valves and the ignition, fuel metering, and other systems, it is necessary to know the exact phase position of the camshafts with respect to the crank mechanism. Even a small offset in phase position, caused e.g. by incorrect installation of the toothed belt or by a tooth jump, results in incorrect mixture control and mismatched ignition angles. An incorrectly installed phase sensor or phase transmitter wheel can also cause the camshafts to be regulated to an incorrect phase position. A phase position offset of this kind not only endangers the internal combustion engine, e.g. because of a possible piston-valve collision, but also, in particular, results in a considerable increase in pollutant emissions. In order to limit pollutant emissions, certain authorities, for example the California Air Resources Board (CARB), therefore require that the camshaft phase positions must be diagnosed and, if applicable, controlled, and that the fault be indicated.
For phase position diagnosis, a method is known from German Published Patent Application No. 40 28 442 in which a rotation speed sensor is disposed on the crankshaft and a phase sensor on the camshaft. The phase position of the camshaft is determined by way of the evaluated sensor signals.
Also known are methods that, with the camshaft phase positioner in a predefined rest position, determine the phase position of the camshaft with respect to the phase position of the crankshaft, and then correct installation tolerances of the phase positioner as applicable.
It has been found in practice, however, that the angular tolerances upon installation of the phase transmitter and the angular tolerances of the rest position of the phase positioner can be quite large. For a camshaft, an installation phase tolerance of 3 to 5° with reference to the camshaft is entirely possible. If phase shifts occur within this tolerance range, for example in the context of a one-tooth offset of a toothed belt, that offset cannot reliably be detected.
The method according to the present invention for determining the phase position of a camshaft of an internal combustion engine, having the features of the independent main claim, has, in contrast, the advantage that the phase position is identified on the basis of operating variables of the air system, and therefore independently of tolerances of a camshaft transmitter wheel or camshaft sensor. In addition, variables originally affected by the incorrect sensing—e.g. the fresh air charge and/or residual gas charge—are directly diagnosed with the method according to the present invention.
It is particularly advantageous that the phase position of the camshaft is identified at at least one operating point with constant rotation speed demand and load demand, thereby improving the reliability with which the phase position is determined. According to a further advantageous refinement, expected operating variables are present for at least one phase position, operating variables of the air system being identified in the at least one phase position at at least one operating point with constant rotation speed demand and load demand, the identified operating variables being compared with the operating variables to be expected in the present phase position, and an incorrectly sensed phase position being inferred in the event of deviations that exceed a limit value.
In a further advantageous embodiment, at least one camshaft is brought into at least two different phase positions, operating variables of the air system being identified, for each phase position that is set, at at least one operating point with a constant rotation speed demand and load demand.
According to a further advantageous refinement, for at least two phase positions, the difference between the operating variables identified in the various phase positions is calculated and is compared with an expected difference. A deviation of the identified difference from the expected difference is ascertained and is compared with a limit value. If the ascertained deviation exceeds that limit value, an erroneously sensed phase position is inferred. By calculating differences it is possible to eliminate systematic errors, for example an offset, thereby advantageously making the diagnosis of phase position more reliable.
According to a further advantageous embodiment, once an erroneously sensed phase position has been detected, the sensing of the phase position or of the camshaft angle is adaptively corrected. This has the advantage that the internal combustion can at first continue to be operated without significant impairment.
According to a further advantageous embodiment, an error message is compulsorily issued, for example to the driver, when the erroneously detected phase position can no longer be adaptively corrected according to predetermined criteria and, for example, target positions of the camshafts can no longer be reached. This has the advantage that larger corrections, which are no longer neutral in terms of operation of the internal combustion engine or, in extreme cases, could result in damage to the engine (piston-valve collision), are signaled in timely fashion.
Further features, potential applications, and advantages of the invention are evident from the description below of exemplifying embodiments of the invention that are depicted in the drawings. All features described or depicted, of themselves or in any combination, constitute the subject matter of the invention, irrespective of their grouping in the claims or their internal references, and irrespective of their presentation and depiction in the description and the claims, respectively.